Recently, attention has been focused on a position detection technology for wireless terminals in the field of sensor networks or cellular communications. For example, in the field of sensor networks, a technology draws attention, which achieves position detection by placing sensors equipped with a wireless communication function in different places and analyzing reception results by these sensors. More specifically, a configuration has been proposed, in which the precision of position detection is increased by considering positions (coordinates), placement density, and radio coverage of the sensors. In the field of cellular communications, a technology is known, which requires Time Difference Of Arrival (TDOA) of radio waves received via multiple receive antennas, and detects the positions based on the TDOA. A system designed to receive radio waves via the multiple receive antennas may include, for example, a Distributed Antenna System (DAS) (see FIG. 2B).
Position detection used in these fields is performed, for example, in the manner shown in FIG. 3. FIG. 3 illustrates a position detection method on the assumption that one wireless terminal X exists in a system with four receiver antennas. In FIG. 3, each of the distances d1˜d4 represents estimated distances calculated based on received radio waves that have arrived at the respective antenna from the wireless terminal X. Because multiple estimated distances can be obtained with use of multiple antennas, an intersection of areas having the estimated distances as their radii may be presumed as a position where the wireless terminal exists. Each estimated distance is calculated based on a receive timing of a received radio wave that has arrived at each antenna.
For example, when a relative delay time difference TDOA between certain antennas is 50 ns, a distance difference between the antennas is calculated as 15 m because light velocity c is approximately 3×108 m/s. When a reference time is determined using the Global Positioning System (GPS), it is also possible to measure a distance based on the time from the reference time until the first radio wave is received. Meanwhile, three-point positioning may be used when position detection is performed using the GPS in a downlink (a link from a base station to a wireless terminal). Even in this case, the first arrived radio wave is used. In order to perform such position detection, it is necessary to obtain a time difference using the first arrived radio wave.
Herein, reference will be made to Japanese Patent No. 3,596,442, which is hereby incorporated by reference.
However, an influence of reflected waves is added to the radio waves in a transmission path. Thus, in order to increase precision of position detection, it is necessary to precisely detect arrival timings of direct waves from received radio waves to which the influence of reflected waves was added in the transmission path. In addition, measurement precision of position detection in the current cellular mobile communication (without GPS function) is 300 m at most. One of the main reasons is that because an occupied bandwidth of cellular terminals is about 1 MHz, time resolution is insufficient, and it is difficult to distinguish direct waves from delayed waves. Besides, if the time resolution is increased in order to make it easy to distinguish direct waves from delayed waves, the wireless terminal needs to transmit broadband sensing signals. Thus, it is very difficult for the wireless terminal to achieve the intention using its limited power resource.